1. Field of the Invention:
The present invention relates to apparatus for measuring low gas pressure or density, more specifically to a hot cathode ionization pressure gauge or manometer.
2. Description of the Prior Art:
For the purpose of measuring neutral gas density in the presence of strong magnetic fields and a high noise background, such as prevail, for example, in plasmaphysical apparatus and fusion experiments, it is known to employ a hot cathode ionization gauge containing a linear triode system with a cathode consisting of a relatively thick (0.6 mm) thoriated tungsten wire, an accelerating grid and a relatively large ion collector which are arranged along an axis at a distance from one another. The axis of the triode system is arranged substantially parallel to the magnetic field, which forces the electrons emitted by the cathode and accelerated by the accelerating grid onto paths which extend along the lines of the magnetic field (Journal of Nuclear Matrials 121 (1984) 151-156). There is known from Journal of Physics E: Scientific Instruments 1974, Vol. 7, pp. 453-457, a hot cathode ionization gauge with a linear electrode system which is surrounded by a plasma shield consisting of a series of spaced, annular plates surrounding the electrode system, which is arranged between two closed end plates.
From Rev. Sci. Instr. 34, No. 8, pp. 942-943, August 1963, there is known a hot cathode ionization gauge with a linear electrode system comprising, in the stated sequence, a plate-shaped collector electrode, a plane screen grid, a helical cathode, a plane modulating grid and a plate-shaped anode. The modulating grid allows the stream of electrons flowing from the cathode to the anode to be interrupted periodically. The grids consist of a tungsten gauze. This and the arrangement of the anode and collector electrode on opposite sides of the cathode (Schulz-Phelps arrangement) make one expect a considerable dependence of the sensitivity on the direction of the magnetic field. Chopping of the emission current of a hot cathode ionization gauge is known from J. Vac. Sci. Technol., Vol. 18, No. 3, April 1981, pp. 1017-1022.